The mechanical behavior of porous sediments is important for the understanding of deformation in accretionary prisms, subsiding and spreading basins, and other settings in which "soft" sediments are subjected to tectonic or gravitational stresses. Unfortunately the relationships among physical properties, mechanical state, and structural fabric for these sediments are very poorly known, which has impeded the development of quantitative geomechanical models. A series of mechanical tests have been made moderately porous (20-40%) silty clays (both natural and artificially consolidated) that will allow us to correlate the physical properties and mechanical state of these sediments with the structural fabrics produced during deformation. We investigated strength surface for laboratory consolidated sediments, as depicted in a 3-dimensional space of effective mean stress, differential stress, and void ratio, at mean stresses typical of geologic conditions in accretionary prisms. For natural sediments from basinal sequences we measured in-situ strength and analyze the failure fabrics produced during bedding parallel shortening. For the study of naturally deformed sediments we have several samples from DSDP sites in the Nankai accretionary prism. Structural fabric analyses included optical microscopy for measurement of the overall and internal geometries of the failure zones, microprobe studies for porosity variations, and x-ray pole figure goniometry for fabric anisotropy and strain.
